1. Field of the Invention
The invention generally relates to a constant velocity joint, and more particularly to a ball-type constant velocity joint having a plurality of balls captured between an inner race and an outer race of the joint.
2. Description of Related Art
In front-wheel drive vehicles and four-wheel drive vehicles, for example, constant velocity joints are generally used in coupling portions between front wheels serving as steering/driving wheels and drive shafts. Such a constant velocity joint allows changes in the joint angle between a driving shaft and a driven shaft connected to the joint, while permitting the driving shaft and the driven shaft to rotate at an equal angular velocity. As one type of the constant velocity joints, a ball-type constant velocity joint is known in which a plurality of balls are captured between an inner race and an outer race of the joint such that torque is transmitted through contact points of the balls with inner walls of grooves formed in the inner and outer races.
The inner race of the ball-type constant velocity joint has an outer spherical surface of a generally spherical shape, and is accommodated in the outer race having an inner spherical surface of a generally spherical shape. In the constant velocity joint, ball grooves extending in the direction of the rotation axis of the joint are formed in the outer spherical surface of the inner race and the inner spherical surface of the outer race at equal intervals in the circumferential direction. The number of the ball grooves formed in each of the inner and outer races is the same as that of the balls. The balls are respectively disposed between the opposed ball grooves of the inner race and the outer race, such that the intervals between the balls as measured in the circumferential direction are fixed or maintained by a cage that is interposed between the outer circumferential surface of the inner race and the inner circumferential surface of the outer race.
With the ball-type constant velocity joint thus constructed, the angle between the rotation axis of the outer race and the rotation axis of the inner race can freely change through movements of the balls along the ball grooves. On the other hand, the displacement of the balls in the circumferential direction is restrained or inhibited by side walls of the ball grooves in the outer spherical surface of the inner race and the inner spherical surface of the outer race, so that relative rotation of the inner race and the outer race is inhibited. With this arrangement, the inner race and the outer race are able to rotate at an equal angular velocity, while permitting changes in the joint angle between the inner and outer races.
As one type of the ball-type constant velocity joint as described above, a so-called UF (undercut free) type constant velocity joint is known in which ball grooves in an open end portion of the outer race are shaped in the form of straight grooves that extend in parallel with the rotation axis of the outer race.
In the UF-type constant velocity joint, the outer race can be formed with its opening having an increased inside diameter without increasing the outside diameter of the outer race. With the outer race thus formed, the joint angle is less likely to be restricted or limited by interference between a shaft connected to the inner race and the inner edge of the open end portion of the outer race, and the joint angle can be accordingly increased.
Japanese Laid-open Patent Publication No. 2001-153149 discloses another example of the UF type constant velocity joint in which the maximum joint angle can be further increased by forming the straight grooves of the outer race so that each groove extends straight in such a direction that the distance between the bottom of the groove and the rotation axis of the outer race increases toward the open end thereof.
However, the known UF-type constant velocity joints may suffer from the following problems caused by the increase in the maximum joint angle.
The outer spherical surface of the inner race and the inner spherical surface of the outer race are formed in substantially uniform arcuate shape as viewed in a plane that contains the rotation axes thereof. If the ball grooves are formed to extend substantially straight in the outer race, the depth of the ball grooves cannot be made constant, and some portions of the ball grooves inevitably have a reduced depth.
Also, the displacement of the balls in the radial directions during rotation of the joint increases as the maximum joint angle increases, and therefore the outside diameter of the cage holding the balls tends to be increased. The increase in the cage diameter requires the inner spherical surface of the outer race to be enlarged, resulting in a reduction in the groove depth of the ball grooves of the outer race as a whole.
In the constant velocity joint as described above, torque is transmitted between the outer race and the inner race via the respective balls to which a load or torque is distributed. It is thus difficult to ensure sufficiently high durability at portions of the outer race in which the ball grooves have a relatively small depth as described above. To ensure sufficiently high durability at these portions, the depth of the overall ball grooves needs to be increased, and the ball diameter also needs to be increased. The increases in the groove depth and ball diameter inevitably result in increases in the size and weight of the constant velocity joint.
The constant-velocity joint as disclosed in the above-identified publication is likely to suffer from the above-described tendencies because of the configuration of the ball grooves.